Non-aqueous dispersions and emulsions are increasingly gaining importance. Especially they are used as electrorheological fluids or compositions that are present as liquid, gels or paste. Under the term electrorheological fluids, one understands dispersions of small-sized particles in hydrophobic and electrically non-conducting oils. The apparent viscosity of these dispersions changes under the influence of an electric constant or alternating field, very quickly and reversibly from the liquid to the plastic or solid state, whereby the current consumption of the ERF shall be as small as possible.
The viscosity increase in an ERF upon application of an electric field is qualitatively to be explained as follows: The colloid-chemically stable dispersed particles polarize in the electric field and agglomerate due to the dipole interaction in the direction of the field lines. This leads to the increase of the apparent viscosity. The agglomeration is reversible: if the electric field is switched off, then the particles re-disperse and the viscosity is reduced to the original value. The electrical polarizability of the disperse phase is thus an important pre-condition or requirement for the establishment of the electrorheological effect. Therefore, ionic or electronically conductive materials are often used as the disperse phase or as an additive thereto.
In a portion of the ERF, which correspond to the state of the art, the disperse phase consists of organic solid substances, such as for example, ion exchange resins (U.S. Pat. No. 3,047,507) or silicone resins (U.S. Pat. No. 5,164,105). However, partially coated inorganic materials, such as e.g. zeolites (U.S. Pat. No. 4,744,914) or silica gel (U.S. Pat. No. 4,668,417) are also used. In the abovementioned substances, the electrorheological effect is to be attributed to the charging or loading of the solid substances with water. Small water contents or proportions increase the ionic conductivity and are thus advantageous for the establishment of the effect. Water-containing systems, however, have a low stability and go along with increased current densities. Solid materials such as partially coated metal powders or zeolites have the disadvantage that they have an abrasive effect. The abrasive wear can be strongly influenced by the selection of the disperse phase. Therefore, polymeric substances, especially elastomers, are preferable to the inorganic powders as the disperse phase e.g. in hydraulic applications. Moreover, homogeneous ERF are known, e.g. from U.S. Pat. No. 5,891,356.
ERF may be utilized everywhere where it is necessary to achieve the transmission of large forces with the aid of small electrical powers, such as e.g. in clutches, hydraulic valves, shock and vibration dampers, brake systems, vibrators, devices for positioning and fixing workpieces, exercise and sport devices or also for medical applications.
Besides the general requirements for an ERF, such as a good electrorheological effect, high temperature stability and chemical resistance, further factors play an important roll in the practical utilization. These include, e.g. the abrasivity, the base viscosity as well as the precipitation stability of the disperse phase. To the extent possible, the disperse phase should not precipitate out as sediment, but should however in each case be well re-dispersable, and even under high mechanical loading should not cause abrasion and should not underlie wear.
An effective electrorheological fluid shall thus have a lowest possible base viscosity, a highest possible shear stress, a lowest possible current uptake, and a high viscosity after application of the electric field, that is to say a large viscosity change or large hydraulic switching index. Moreover an effective ERF shall be utilizable over a wide temperature range (approximately −30° C. to approximately +150° C.) and comprise an excellent material tolerability.
As is known, the ER effect increases with the volume proportion of the disperse phase. Achieving a low base viscosity with a high solid material content or proportion is dependent on first the form or shape as well as the particle size distribution of the disperse phase and secondly the dispersion effect of possibly utilized dispersing auxiliary agents (see e.g. EP 2016117). Additionally, the conductivity of the disperse phase is also dependent on the particle size. The optimization of all properties of the ERF is only possible in connection with the exact adjustment or setting of the particle size or the particle size distribution of the disperse phase.
The abovementioned ERF corresponding to the state of the art are generally produced by dispersing a solid material into a dispersion medium such as e.g. halogen-free or halogenated hydrocarbons, aromatics or silicone oils. In that regard, the viscosity of the resulting suspension depends on the form or shape and the size or the size distribution of the dispersed particles, as well as the solid material concentration and the dispersion effect of possibly utilized dispersing auxiliary agents such as dispersion stabilizers. High volume-referenced solid material contents with low viscosity are only achievable with difficulty when using non-spherical particles.
However, in practice it has been found to be disadvantageous that the use of salts as an additive in the use of such ERF can lead to an undesired corrosion of the electrodes, which has a disadvantageous effect on the electrorheological effect and on the durability of the components.
Thus it is suggested in the patent application DE 10 2009 048 825 A1, basically to avoid the use of salt doping in ERF to achieve a corrosion-inhibiting effect in the use of ERFs. There it is suggested to use organic non-ionic doping agents.
The patent EP 0 567 649 B1 is also concerned with the problem of corrosion avoidance in the use of ERFs. There it is suggested to solve the problem through the use of corrosion inhibitors.